CH4 gas feeds on performance of mixed matrix membranes using carbon molecular sieves

Journal of Membrane Science 221 (2003) 233–239

Short communication

Effect of condensable impurity in CO2 /CH4 gas feeds on performance of mixed matrix membranes using carbon molecular sieves De Q. Vu a,c , William J. Koros b,∗ , Stephen J. Miller c b

a Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712-1062, USA School of Chemical Engineering, Georgia Institute of Technology, 778 Atlantic Drive NW, Atlanta, GA 30332-0100, USA c ChevronTexaco Energy Research and Technology Company, 100 Chevron Way, Richmond, CA 94802-0627, USA

Received 17 March 2003; accepted 28 May 2003

Abstract The permeation properties of a mixed matrix membrane film were investigated with 10% CO2 /90% CH4 gas feeds containing a vapor impurity of toluene (70 ppm). The mixed matrix film was comprised of fine particles of high-selective carbon molecular sieves (CMS) dispersed within a glassy polyimide matrix (Matrimid® 5218). In preceding publications, it was demonstrated that the carbon molecular sieve (CMS) particles produced a mixed matrix membrane having significantly enhanced effective permselectivities (CO2 /CH4 and O2 /N2 ) over the intrinsic properties of the neat polymer matrix phase alone. In this paper, a Matrimid® -CMS mixed matrix flat film (19 vol.% CMS) was exposed to a 10% CO2 /90% CH4 /70 ppm toluene gas feed at 500 psia (34.5 bara) and 35 ◦ C for a period of up to 60 h. The identical experiment was performed on a pure Matrimid® polymer film for comparison. The results of this short-term study indicate that both the Matrimid® mixed matrix film and the pure Matrimid® film exhibit fairly stable properties in the presence of the low-concentration toluene impurity. The Matrimid® -CMS mixed matrix film (19 vol.% CMS) sustains its CO2 /CH4 selectivity enhancement (∼13%) over the intrinsic CO2 /CH4 selectivity of the pure Matrimid® polymer film. As speculated in a previous publication on CMS fibers, larger-sized impurities, such as toluene, may only be successful in blocking or occupying the larger, non-selective pores of the CMS particles and may not access the smaller, selective pores that are accessible to CO2 . In the presence of the binary mixture of 10% CO2 /90% CH4 , permeabilities approach a stable steady state within a few hours for both the pure Matrimid® polymer film and the mixed matrix sample. For the pure Matrimid® polymer film, the CO2 permeability in the presence of the ternary mixture is slightly lower, but barely distinguishable beyond experimental uncertainty. On the other hand, the mixed matrix sample shows a more protracted approach to steady state in the presence of the toluene impurity, which ultimately stabilizes after roughly 60 h. We hypothesize that surface adsorption of the toluene molecules onto the CMS particles may induce a slow relaxation leading to better packing at the interface between the carbon and matrix reflected by the protracted changes seen. © 2003 Elsevier B.V. All rights reserved. Keywords: Mixed matrix membrane; Heterogeneous membrane; Carbon molecular sieve; Gas separation; Natural gas

1. Introduction ∗

Corresponding author. Tel.: +1-404-385-2845; fax: +1-404-894-2866. E-mail address: [email protected] (W.J. Koros).

The presence of heavy, condensable hydrocarbons can present problems for polymeric, asymmetric hollow fibers. Such impurities can be particularly

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0376-7388(03)00245-X

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prevalent in natural gas streams. In the past few decades, membranes have been investigated for applications in CO2 removal in natural gas processing streams and have displayed very attractive separation properties in the laboratory [1–6]. However, loss of flux and/or selectivity due to heavy hydrocarbon exposure can be detrimental to membrane performance in the field. Some limited studies have demonstrated this potential problem for current membrane materials [7,8]. As a result, it is important to evaluate separation performance of any new membrane material under these adverse conditions. Hybrid membrane materials, or mixed matrix membranes, are a new class of membrane materials that offer the potential of significantly advancing the current technology. This concept involves the incorporation of highly selective molecular sieves within a polymeric matrix, resulting in a hybrid membrane material with superior selectivity over that of polymer matrix alone, while maintaining processability under current membrane formation technology [9,10]. Recently, we reported very attractive CO2 /CH4 separation properties for mixed matrix membrane films comprising highly selective carbon molecular sieve (CMS) particles dispersed within glassy polymer matrices [11,12]. For the CO2 /CH4 separation, enhancements by as much as 45% in CO2 /CH4 permselectivity and 200% in CO2 permeability over the corresponding intrinsic permeation properties of the pure polymer matrix phases were observed. These performance enhancements increased as the loading of CMS particles (up to 35% by weight) being dispersed within the two polymer matrices (Matrimid® 5218 and Ultem® 1000) increased. As discussed earlier, it is important to evaluate performance of these materials in the presence of condensable, heavy hydrocarbons. This paper reports preliminary experimental results of one of these mixed matrix membrane films when exposed to a mixed gas (10% CO2 /90% CH4 ) stream containing a toluene impurity (70 ppm) as a representative condensable, aromatic hydrocarbon.

2. Background Since mixed matrix membranes for gas separations are relatively new, no research work examining their performance under condensable hydrocarbon vapor

exposure has been reported in the open literature. However, there has been limited research work investigating the effects of heavy hydrocarbon vapor impurities in natural gas (CO2 /CH4 ) feeds on the membrane performance of polymeric hollow fibers and pure CMS hollow fibers. The presence of such condensable vapor impurities has been shown to have an especially negative effect on polymeric membranes. One study examining glassy polymeric (polyimide) membrane films reports harsh performance declines in the range of 50% reduction in CO2 /CH4 selectivity accompanied with plasticization due to small, but saturated, concentrations of toluene or n-hexane in mixed gas feeds of CO2 /CH4 [7]. Similar work [13] with 100 and 300 ppm of toluene and n-heptane in 10% CO2 /90% CH4 gas feeds shows similar selectivity losses (∼30–50%) for asymmetric polyimide hollow fibers. Also, Tanihara et al. [14], investigating toluene impurity concentrations of 1600–7600 ppm in equimolar H2 /CH4 mixtures, show that polyimide membranes experience H2 /CH4 selectivity losses as great at 85% from the original impurity-free mixture. In addition to selectivity losses, reductions in flux (∼30–60%) were also observed and were attributed to competition effects and possible compaction of the transition layer in the asymmetric hollow fiber structure [13,14]. It is postulated that high-selectivity polyimide membrane materials may be susceptible to heavy hydrocarbon exposure because of their greater reliance on diffusivity selectivity [7]. Thus, disruption of polymer chain mobility possibly caused by condensable penetrants can play a more detrimental role in diffusivity–selectivity polymers. Consequently, condensable hydrocarbons have been suspected of causing membrane performance declines in the field as well [8,13,15,16]. As is the case with glassy polymeric membranes, the effect of these condensable agents or impurities in pure CMS membranes is an important concern. In a recent study of CMS hollow fiber membranes [17], toluene and n-heptane were separately used as representative aromatic and paraffinic impurities, respectively, in 10% CO2 /90% CH4 gas feeds streams containing up to 300 ppm of these impurities at shell-side feed pressures of up to 900 psia and temperatures of 35 and 50 ◦ C. In this work, Vu et al. show that the CMS membranes maintained CO2 /CH4 selectivity with only 7–20% reduction in CO2 permeance during exposure to gas feeds containing these impurities, in

D.Q. Vu et al. / Journal of Membrane Science 221 (2003) 233–239

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ence, Gibbstown, NJ) to dissolve Matrimid® 5218 to prepare 5–10 wt.% polymer solution. The Matrimid® 5218 films were cast in a circular ring mold or with a uniform-thickness film applicator (Paul N. Gardner Co., Pompano Beach, FL) on smooth glass surface. After slow evaporation at room temperature (approx. 12 h), the dry films were generally 2 mils (1 mil = 0.001 in.) in thickness. To remove residual solvent, the films were heated in a vacuum oven at 100 ◦ C overnight. The polymeric precursor films were pyrolyzed in a quartz tube furnace. The detailed description of the pyrolysis procedure used is reported elsewhere [18,19]. After pyrolysis, the CMS films were further milled into very fine particles (submicron to micron) by a ball mixer/mill (SPEX Model 8000, Metuchen, NJ) [11]. After milling, the fine CMS particles were heated to 250 ◦ C under vacuum for at least overnight prior to any use or characterization. The purpose of this heat treatment was to remove moisture that may have been sorbed by the CMS particles during storage. As a result of the milling, the distribution of particle sizes typically ranged from submicron to 10 ␮m in size, as verified by scanning electron microscopy (SEM). Mixed matrix films were prepared by solution casting a slurry of fine CMS particles dispersed within a polymer solution. Generally, the slurry concentration was about 15–20 wt.% solids (CMS particles and polymer) in solvent. For the results presented in this paper, Matrimid® 5218 was used as the polymer matrix phase in the mixed matrix membrane. The detailed procedure to prepare the mixed matrix films and the analytical and experimental techniques used to characterize them are discussed in a preceding work [11]. The performance of the synthesized mixed matrix membrane films was evaluated with pure gas (O2 ,

comparison to “clean” gas feeds without these impurities. It is postulated that the impurity penetrants are only effective in entering and blocking non-selective micropores and are not able to enter the smaller, selective ultramicropores. Furthermore, a simple in situ regeneration procedure of moderate heating (70–90 ◦ C) with dry N2 purge gas resulted in complete or almost complete recovery of CO2 permeance without loss of CO2 /CH4 selectivity. This evidence further supports the hypothesis that only physisorption of the impurity molecules may be occurring. Like Vu et al., Tanihara et al. [14] report similar findings with CMS fibers using higher toluene concentrations (1600–7600 ppm) for the H2 /CH4 separation.

3. Experimental Preparation of the CMS mixed matrix membrane films involved three primary steps: (1) generation of sieve particles, (2) preparation of the polymer– sieve slurry mixture, and (3) casting the polymer–sieve mixture to form a mixed matrix membrane film. The carbon molecular sieve material used (referenced as CMS 800-2) was formed from the vacuum pyrolysis of Matrimid® 5218 (Vantico Inc., Luxembourg). Matrimid® 5218 is a commercially available polyimide made from the monomers 3,3 ,4,4 -benzophenone tetracarboxylic dianhydride and diaminophenylindane and is currently used as a gas separation membrane material. The chemical structure and physical properties of Matrimid® 5218 are shown in Table 1. Before pyrolysis, the Matrimid® precursor material was first prepared as a dense, flat polymer film from solution casting using dichloromethane (CH2 Cl2 ) solvent (chromatographic grade, EM Sci-

Table 1 Chemical structure and physical properties of Matrimid® 5218, which was used as the precursor material pyrolyzed to generate the carbon molecular sieve (CMS) particles and as the polymer matrix phase in mixed matrix membranes Polymer

Chemical structure H3C

Matrimid® 5218

CH3

O N

O

Tg (◦ C)

1.24

302

O N

H3C O

Density (g/cm3 )

O

n

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D.Q. Vu et al. / Journal of Membrane Science 221 (2003) 233–239

N2 , CO2 , and CH4 ) and mixed gas (10% CO2 /90% CH4 ) permeation experiments. Using the manometric (or constant volume) method [11], permeability measurements were performed at 35 ◦ C. For permeation experiments using the 10% CO2 /90% CH4 gas mixture (Air Liquide, certified grade), the permeate stream was connected to a gas chromatograph (Hewlett-Packard 5890 Series II, Atlanta, GA) having a thermal conductivity detector (TCD) with a packed column for CO2 /CH4 separation (HayeSep Q 80/100 mesh, Hewlett-Packard). The retentate (or residue) flow rate was controlled by a metering valve and was monitored by a digital bubble flow meter. This retentate flow rate was set so that the permeate flow rate was less than 1% of the feed flow rate. This condition specifies that the “stage cut” is less than 1%, which ensures that the feed or upstream composition does not vary during the permeation run. The composition of the permeate stream was determined by the gas chromatograph. The permeability coefficients for CO2 and CH4 were calculated on a fugacity driving force with the fugacity coefficients of CO2 and CH4 in the binary mixture computed from the virial equation of state [20] using the pure-component second virial coefficients [21] and the second virial coefficient for the binary mixture [20].

4. Results and discussion A Matrimid® mixed matrix film (19 vol.% CMS) sample was studied in the presence of a 10% CO2 /90% CH4 mixed gas feed containing an impurity of toluene (70.4 ppm). This Matrimid® mixed matrix film was previously prepared and examined with pure gas permeation experiments at 35 ◦ C [11,12]. The pure

gas permeation data (permeabilities and O2 /N2 and CO2 /CH4 permselectivities) are presented in Table 2. The permeation properties of the pure continuous Matrimid® polymer phase and the pure disperse sieve phase (CMS 800-2) forming the mixed matrix film are also shown for comparison. The sample achieved steady state permeation behavior within 10 h in the case of the pure component and binary gas feeds. No hysteresis was seen upon pressurization and depressurization up to 500 psia (34.5 bara) feed pressure for the mixed gas or for the pure gases at partial pressures equivalent to their values in the feed (e.g., 50 psia CO2 , 450 psia CH4 ). After evacuating for at least 24 h, the membrane film was exposed to 500 psia (34.5 bara) mixed gas feed mixture containing 70.4 ppm toluene at 35 ◦ C. Continuous flow was maintained with the retentate stream set for <1% stage cut. Permeation flux measurements were periodically taken with a stripchart recorder, and permeate samples analyzed by gas chromatography. The first sample was measured approximately 2 h after introduction of the pre-heated feed gas. Fig. 1 presents the results as a function of elapsed time of exposure. For comparison, a pure Matrimid® polymer film, characterized in the same manner as the Matrimid® mixed matrix film, is also shown. The results of this short-term study indicate that both the Matrimid® -CMS mixed matrix film and the pure Matrimid® film exhibit quite stable properties in the presence of the low-concentration toluene impurity. As noted above, the binary mixture of 10% CO2 /90% CH4 approached a stable steady state within a few hours for both the pure Matrimid® polymer film and the mixed matrix sample. The CO2 permeability in the presence of the ternary mixture is slightly lower, but barely distinguishable beyond experimental uncertainty.

Table 2 Permeation properties of a mixed matrix film containing 19 vol.% loading of carbon molecular sieve inserts (CMS 800-2) in Matrimid® 5218 matrix and the intrinsic permeation properties of the Matrimid® and CMS 800-2 phasesa Permeability (Barrer)

Matrimid®

Continuous phase: 5218 Disperse phase: CMS 800-2 19 vol.% CMS in Matrimid® 5218 a

Permselectivity

CO2

CH4

O2

N2

CO2 /CH4

O2 /N2

10.0 44.0 10.6

0.28 0.22 0.23

2.12 22.0 2.41

0.32 1.65 0.35

35.3 200 46.7

6.6 13.3 7.0

Pure gas permeation measurements with 50 psia upstream for CO2 , CH4 , O2 , and N2 ; temperature: 35 ◦ C.

MatrimidÆ Mixed Matrix Film (10% CO2/90% CH4 with 70.4 ppm toluene ) Pure MatrimidÆ (10% CO2/90% CH4 with 70.4 ppm toluene) MatrimidÆ Mixed Matrix Film (10% CO2/90% CH4) Pure MatrimidÆ (10% CO2/90% CH4)

60

CO2/CH4 Permselectivity

CO2 Permeability (Barrer)

9 8

7

6

Evacuation of upstream and downstream for 12 hours prior to re-introduction of 10% CO2/90% CH4 feed gas

Feed Pressure = 500 psia Temperature = 35∞C

MatrimidÆ Mixed Matrix Film (10% CO2/90% CH4 with 70.4 ppm toluene ) Pure MatrimidÆ (10% CO2/90% CH4 with 70.4 ppm toluene) MatrimidÆ Mixed Matrix Film (10% CO2/90% CH4) Pure Matrimid Æ (1 0% CO 2 /9 0% C H 4 )

50

40 Evacuation of upstream and downstream for 12 hours prior to re-introduction of 10% CO2/90% CH4 feed gas

Feed Pressure = 500 psia Temperature = 35∞C

5

30 0

20

40

60

Elapsed Time (hours)

80

100

0

20

40

60

80

100

E lapsed T ime (h ours)

Fig. 1. Membrane performance of a Matrimid® mixed matrix film (19 vol.% CMS) and a pure Matrimid® film in the presence of a continuous 10% CO2 /90% CH4 feed gas mixture containing 70.4 ppm toluene at 500 psia (34.5 bara) and 35 ◦ C over an elapsed time of 60–80 h.

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10

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In contrast to the pure Matrimid® polymer film, the mixed matrix sample shows a more protracted approach to steady state in the presence of the toluene impurity in the ternary mixture. The response ultimately stabilizes after roughly 60 h. This protracted response could reflect the slow establishment of steady state permeation of the larger toluene molecule. In such a situation, however, one would expect this protracted response to be also apparent for the pure Matrimid® film if this were the key issue involved. In any case, the mixed matrix selectivity enhancement, ∼13% higher compared to the pure Matrimid® film sample, is maintained during this protracted process, since both CH4 and CO2 permeabilities undergo similar reductions. As speculated in a previous publication on CMS fibers [17], larger-sized impurities, such as toluene, may only be successful in blocking or occupying the larger, non-selective pores of the CMS particles and may not access the smaller, selective pores that are accessible to CO2 . Evacuation and retesting with the binary mixture of 10% CO2 /90% CH4 produces no apparent recovery of the permeabilities or changes in selectivities (see arrows in Fig. 1). We hypothesize that surface adsorption of the toluene molecules onto the CMS particles may induce a slow relaxation leading to better packing at the interface between the carbon and matrix reflected by the protracted changes seen.

5. Conclusions Although the results reported here show promising performance stability of CMS mixed matrix membranes, the results would have been more convincing if the testing had been at conditions where the neat Matrimid® film is adversely affected. Further study of mixed matrix membrane performance under mixed gas feeds containing condensable vapor components is necessary to fully evaluate the robustness of mixed matrix membranes. Future work should investigate higher activity levels of impurities, other different types of impurities, and longer periods of time to characterize fully their long-term, steady state performance. Also, the presence of water vapor in these feed streams should also be examined since water vapor is known to negatively affect CMS materials. In addition to water vapor, entrained glycol impurities resulting from glycol units used to dry natural gas streams (and

added to natural gas streams to prevent hydrate formation) should also be examined for their impact on membrane performance. These studies could identify the potential impact of these impurities and may require additional research work to investigate possible preventive measures, such as pre-treatment of process streams, pre-treatment of membrane materials (e.g., preventive layers like Teflon® AF), or post-treatment steps (e.g., regeneration). It should be emphasized that the experiments reported here were performed only on dense films. In commercial asymmetric membranes (e.g., hollow fibers), it is hypothesized that one of the main flux loss mechanisms may be attributable to such phenomena as physical ageing and compaction that can cause morphological changes in the ultrathin selective “skin”. So, beyond dense film testing reported here, testing of membranes having thin-skinned asymmetric geometries is ultimately necessary to provide a complete evaluation of membrane performance in the presence of impurities.

Acknowledgements The authors gratefully acknowledge the financial support of the ChevronTexaco Energy Research and Technology Company (Richmond, CA) and the Separations Research Program (SRP) at The University of Texas at Austin. This material is based upon work supported under a National Science Foundation Graduate Research Fellowship (D.Q. Vu).

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